The White House panel investigating the Gulf Coast oil spill says BP and one of its main contractors, Halliburton, ignored critical design flaws weeks before the explosion at the Macondo well. On Thursday, the National Oil Spill Commission said multiple tests showed the cement slurry used to seal the well was unstable and unlikely to hold. The companies still went ahead with their cementing work despite the findings. Halliburton has previously claimed the tests had proved the cement’s reliability.